A widely used technique for searching for hydrocarbons, e.g., oil and/or gas, is the seismic exploration of subsurface geophysical structures. Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine-based seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process.
The seismic exploration process includes generating seismic waves (i.e., sound waves) directed toward the subsurface area, gathering data on reflections of the generated seismic waves at interfaces between layers of the subsurface, and analyzing the data to generate a profile (image) of the geophysical structure, i.e., the layers of the investigated subsurface. This type of seismic exploration can be used both on the subsurface of land areas and for exploring the subsurface of the ocean floor.
Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth, by first generating the energy waves in or on the ocean. By measuring the time it takes for the reflections to come back to one or more receivers (usually very many, perhaps on the order of several hundreds, or even thousands), it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
Seismic waves are initiated by a source, follow one or more paths based on reflection and refraction until a portion of the seismic waves are detected by one or more receivers. Upon detection, data associated with the seismic waves is recorded and then processed for producing an accurate image of the subsurface. The processing can include various phases, e.g., velocity model determination, prestack, migration, poststack, etc., which are known in the art and thus their description is omitted here.
A traditional marine system for recording seismic waves is illustrated in FIG. 1, and this system is described in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, a plurality of seismic receivers 2 are each removably attached to a pedestal 4 together with a memory device 6. A plurality of such receivers is deployed on the bottom 8 of the ocean. A source vessel 10 tows a seismic source 12 that is configured to emit seismic wave 14. Seismic wave 14 propagates downward, toward the ocean bottom 8. After being reflected from a structure 16, the seismic wave (primary) is recorded (as a trace) by the seismic receiver 2.
Multi-component marine acquisition uses receivers that are capable of measuring a pressure wavefield and at least one component of a particle motion that is associated with acoustic signals. Examples of particle motions include one or more components of a particle displacement, one or more components of a particle velocity (for example, inline (X), crossline (Y) and vertical (Z) components) and one or more components of a particle acceleration.
Noise can come from a variety of sources which can affect the different components of the received seismic signals in different manners. In most cases the noise characteristic is different for each component in term of coherency, location or amplitude. As all components contain valuable information it is desirable to design efficient methods that de-noise each component independently and/or simultaneously. Standard de-noising techniques either rely on the noise being incoherent (f-x-deconvolution, projection filtering, etc.) or that the noise is distinguishable in some other way (e.g., Radon demultiple discrimination on moveout). Multi volume de-noising will use the coherency of the signal between components as a discriminator to de-noise the data.
Land reflection seismology involves deploying one or more seismic sources and seismic detectors at predetermined locations. The seismic sources generate pressure (seismic) waves, which propagate into the geological formations. Changes in acoustic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the seismic sources is reflected inside the geological formation.
In a land data acquisition system, the detectors may be arranged along receiver lines, while the seismic sources are usually positioned at shot points in-between the receiver lines on shot lines parallel to the receiver lines as the land topography allows. The recorded seismic data corresponds to signals (due to seismic wave reflections inside the geological formation) and to overlapping noise. The seismic data includes values proportional to pressure versus time or to displacement versus time as sensed by seismic detectors (e.g., hydrophones and geophones), associated with the corresponding positions of the detectors and of the shot point.
De-noising techniques balance the efficiency of the de-noising and the preservation of the underlying signal. In practice it is often more important to protect the signal so that after processing, genuine information can be extracted. This can significantly reduce the de-noising efficiency. For that reason modern de-noising techniques try to differentiate signal and noise by including as many signal characteristics as possible such, as amplitude and directions, in order to discriminate the noise from the signal.
Accordingly, it would be desirable to provide methods and systems that avoid the afore-described problems and drawbacks.